Limitations to oxygen diffusion and equilibration in in vitro cell exposure systems in hyperoxia and hypoxia

Enzyme-induced hypoxia leads to inflammation in urothelial cells in vitro

PURPOSE

To determine the contributions of different durations of hypoxia to NLRP3 inflammasome activation in urothelial cells and how ischemic changes in bladder tissues is an important chemical que that leads to pathological changes seen in BOO.

METHODS

A rat urothelial cell line (MYP3) was exposed to either a short duration (2 h) or long duration (6 h) of enzyme-induced hypoxia. Following exposure to a short duration of hypoxia, NO and ATP concentrations were measured from supernatant media and caspase-1 levels were measured from cell lysates. In a separate experiment, cells were fixed following hypoxia exposure and immunostained for HIF-1α stabilization.

RESULTS

Although short exposure of low oxygen conditions resulted in a hypoxic response in MYP3 cells, as indicated by HIF-1α stabilization and increased NO activity, NLRP3 inflammasome activation was not observed as caspase-1 activity remained unchanged. However, exposure of MYP3 cells to a longer duration of hypoxia resulted in an increase in intracellular caspase-1 activity. Furthermore, treatment with antioxidant (GSH) or TXNIP inhibitor (verapamil) attenuated the hypoxia-induced increase in caspase-1 levels indicating that hypoxia primarily drives inflammation through a ROS-mediated TXNIP/NLRP3 pathway.

CONCLUSION

We conclude that hypoxia induced bladder damage requires a duration that is more likely related to elevated storage pressures/hypoxia, seen in later stages of BOO, as compared to shorter duration pressure elevation/hypoxia that is encountered in normal micturition cycles or early in the BOO pathology where storage pressures are still normal.

Suspended hydrogel culture as a method to scale up intestinal organoids

Primary tissue-derived epithelial organoids are a physiologically relevant in vitro intestinal model that have been implemented for both basic research and drug development applications. The existing method of culturing intestinal organoids in surface-attached native extracellular matrix (ECM) hydrogel domes is not readily amenable to large-scale culture and contributes to culture heterogeneity. We have developed a method of culturing intestinal organoids within suspended basement membrane extract (BME) hydrogels of various geometries, which streamlines the protocol, increases the scalability, enables kinetic sampling, and improves culture uniformity without specialized equipment or additional expertise. We demonstrate the compatibility of this method with multiple culture formats, and provide examples of suspended BME hydrogel organoids in downstream applications: implementation in a medium-throughput drug screen and generation of Transwell monolayers for barrier evaluation. The suspended BME hydrogel culture method will allow intestinal organoids, and potentially other organoid types, to be used more widely and at higher throughputs than previously possible.

Importance of oxygen tension in human ovarian tissue in vitro culture

STUDY QUESTION

Is there any difference between 20% and 5% oxygen (O2) tension in vitro culture (IVC) on the viability and quality of human follicles contained in cultured ovarian cortex?

SUMMARY ANSWER

An O2 tension of 5% yields higher follicle viability and quality than does 20% O2 tension after 6 days of IVC.

WHAT IS KNOWN ALREADY

The primordial follicle (PMF) pool resides within the ovarian cortex, where the in vivo O2 tension ranges between 2% and 8%. Some studies suggest that lowering O2 tension to physiological levels may improve in vitro follicle quality rates.

STUDY DESIGN, SIZE, DURATION

This prospective experimental study included frozen-thawed ovarian cortex from six adult patients (mean age: 28.5 years; age range: 26-31 years) who were undergoing laparoscopic surgery for non-ovarian diseases. Ovarian cortical fragments were cultured for 6 days at (i) 20% O2 with 5% CO2 and (ii) 5% O2 with 5% CO2. Non-cultured fragments served as controls.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Cortical fragments were used for the following analyses: hematoxylin and eosin staining for follicle count and classification; Ki67 staining to evaluate PMF proliferation; cleaved caspase-3 immunostaining to identify follicle apoptosis; 8-hydroxy-2-deoxyguanosine and gamma-H2AX (γH2AX) immunolabeling to detect oxidative stress damage and DNA double-strand breaks (DSBs) in oocytes and granulosa cells (GCs); and β-galactosidase staining to assess follicle senescence. Droplet digital PCR was also performed to further explore the gene expression of superoxide dismutase 2 (SOD2) and glutathione peroxidase 4 (GPX4) from the antioxidant defense system and cyclin-dependent kinase inhibitors (p21 and p16) as tissue senescence-related genes.

MAIN RESULTS AND THE ROLE OF CHANCE

Apoptosis (P = 0.002) and follicle senescence (P < 0.001) rates were significantly lower in the 5% O2 group than in the 20% O2 group. Moreover, GCs in follicles in the 20% O2 group exhibited significantly (P < 0.001) higher oxidative stress damage rates than those in the 5% O2 group. DNA DSB damage rates in GCs of follicles were also significantly higher (P = 0.001) in the 20% O2 group than in the 5% O2 group. SOD2 expression was significantly greater in the 5% O2 group compared to the 20% O2 group (P = 0.04) and the non-cultured group (P = 0.002). Expression of p21 was significantly increased in both the 20% O2 (P = 0.03) and 5% O2 (P = 0.008) groups compared to the non-cultured group. Moreover, the 20% O2 group showed significantly greater p16 expression (P = 0.04) than the non-cultured group, while no significant variation was observed between the 5% O2 and no culture groups.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This study focuses on improving follicle outcomes during the first step of ovarian tissue IVC, where follicles remain in situ within the tissue. The impact of O2 tension in further steps, such as secondary follicle isolation and maturation, was not investigated here.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings suggest that 5% O2 tension culture is a promising step toward potentially solving the problem of poor follicle viability after IVC.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by grants from the Fonds National de la Recherche Scientifique de Belgique (FNRS-PDR T.0064.22, CDR J.0063.20 and grant 5/4/150/5 awarded to M.M.D.). The authors have nothing to disclose.

Fast cycling of intermittent hypoxia in a physiomimetic 3D environment: A novel tool for the study of the parenchymal effects of sleep apnea

Background: Patients with obstructive sleep apnea (OSA) experience recurrent hypoxemic events with a frequency sometimes exceeding 60 events/h. These episodic events induce downstream transient hypoxia in the parenchymal tissue of all organs, thereby eliciting the pathological consequences of OSA. Whereas experimental models currently apply intermittent hypoxia to cells conventionally cultured in 2D plates, there is no well-characterized setting that will subject cells to well-controlled intermittent hypoxia in a 3D environment and enable the study of the effects of OSA on the cells of interest while preserving the underlying tissue environment. Aim: To design and characterize an experimental approach that exposes cells to high-frequency intermittent hypoxia mimicking OSA in 3D (hydrogels or tissue slices). Methods: Hydrogels made from lung extracellular matrix (L-ECM) or brain tissue slices (300-800-μm thickness) were placed on a well whose bottom consisted of a permeable silicone membrane. The chamber beneath the membrane was subjected to a square wave of hypoxic/normoxic air. The oxygen concentration at different depths within the hydrogel/tissue slice was measured with an oxygen microsensor. Results: 3D-seeded cells could be subjected to well-controlled and realistic intermittent hypoxia patterns mimicking 60 apneas/h when cultured in L-ECM hydrogels ≈500 μm-thick or ex-vivo in brain slices 300-500 μm-thick. Conclusion: This novel approach will facilitate the investigation of the effects of intermittent hypoxia simulating OSA in 3D-residing cells within the parenchyma of different tissues/organs.

Environmental oxygen affects ex vivo growth and proliferation of mesenchymal progenitors by modulating mitogen-activated protein kinase and mammalian target of rapamycin signaling

BACKGROUND AIMS

Stem and progenitor cells of hematopoietic and mesenchymal lineages reside in the bone marrow under low oxygen (O₂) saturation. O₂ levels used in ex vivo expansion of multipotent mesenchymal stromal cells (MSCs) affect proliferation, metabolism and differentiation.

METHODS

Using cell-based assays and transcriptome and proteome data, the authors compared MSC cultures simultaneously grown under a conventional 19.95% O₂ atmosphere or at 5% O₂.

RESULTS

In 5% O₂, MSCs showed better proliferation and higher self-renewal ability, most probably sustained by enhanced signaling activity of mitogen-activated protein kinase and mammalian target of rapamycin pathways. Non-oxidative glycolysis-based energy metabolism supported growth and proliferation in 5% O₂ cultures, whereas MSCs grown under 19.95% O₂ also utilized oxidative phosphorylation. Cytoprotection mechanisms used by cells under 5% O₂ differed from 19.95% O₂  suggesting differences in the triggers of cell stress between these two O₂  conditions.

CONCLUSIONS

Based on the potential benefits for the growth and metabolism of MSCs, the authors propose the use of 5% O₂ for MSC culture.

Experimental Models to Study End-Organ Morbidity in Sleep Apnea: Lessons Learned and Future Directions

Sleep apnea (SA) is a very prevalent sleep breathing disorder mainly characterized by intermittent hypoxemia and sleep fragmentation, with ensuing systemic inflammation, oxidative stress, and immune deregulation. These perturbations promote the risk of end-organ morbidity, such that SA patients are at increased risk of cardiovascular, neurocognitive, metabolic and malignant disorders. Investigating the potential mechanisms underlying SA-induced end-organ dysfunction requires the use of comprehensive experimental models at the cell, animal and human levels. This review is primarily focused on the experimental models employed to date in the study of the consequences of SA and tackles 3 different approaches. First, cell culture systems whereby controlled patterns of intermittent hypoxia cycling fast enough to mimic the rates of episodic hypoxemia experienced by patients with SA. Second, animal models consisting of implementing realistic upper airway obstruction patterns, intermittent hypoxia, or sleep fragmentation such as to reproduce the noxious events characterizing SA. Finally, human SA models, which consist either in subjecting healthy volunteers to intermittent hypoxia or sleep fragmentation, or alternatively applying oxygen supplementation or temporary nasal pressure therapy withdrawal to SA patients. The advantages, limitations, and potential improvements of these models along with some of their pertinent findings are reviewed.

Ion modulatory treatments toward functional self-assembled neocartilage

Signals that recapitulate in vitro the conditions found in vivo, such as hypoxia or mechanical forces, contribute to the generation of tissue-engineered hyaline-like tissues. The cell regulatory processes behind hypoxic and mechanical stimuli rely on ion concentration; iron is required to degrade the hypoxia inducible factor 1a (HIF1α) under normoxia, whereas the initiation of mechanotransduction requires the cytoplasmic increase of calcium concentration. In this work, we propose that ion modulation can be used to improve the biomechanical properties of self-assembled neocartilage constructs derived from rejuvenated expanded minipig rib chondrocytes. The objectives of this work were 1) to determine the effects of iron sequestration on self-assembled neocartilage constructs using two doses of the iron chelator deferoxamine (DFO), and 2) to evaluate the performance of the combined treatment of DFO and ionomycin, a calcium ionophore that triggers cytoplasmic calcium accumulation. This study employed a two-phase approach. In Phase I, constructs treated with a high dose of DFO (100 µM) exhibited an 87% increase in pyridinoline crosslinks, a 57% increase in the Young's modulus, and a 112% increase in the ultimate tensile strength (UTS) of the neotissue. In Phase II, the combined use of both ion modulators resulted in 150% and 176% significant increases in the Young's modulus and UTS of neocartilage constructs, respectively; for the first time, neocartilage constructs achieved a Young's modulus of 11.76±3.29 MPa and UTS of 4.20±1.24 MPa. The results of this work provide evidence that ion modulation can be employed to improve the biomechanical properties in engineered neotissues. STATEMENT OF SIGNIFICANCE: The translation of tissue-engineered products requires the development of strategies capable of producing biomimetic neotissues in a replicable, controllable, and cost-effective manner. Among other functions, Fe2+ and Ca2+ are involved in the control of the hypoxic response and mechanotransduction, respectively. Both stimuli, hypoxia and mechanical forces, are known to favor chondrogenesis. This study utilized ion modulators to improve the mechanical properties self-assembled neocartilage constructs derived from expanded and rejuvenated costal chondrocytes via Fe2+ sequestration and Ca2+ influx, alone or in combination. The results indicate that ion modulation induced tissue maturation and a significant improvement of the mechanical properties, and holds potential as a tool to mitigate the need for bioreactors and engineer hyaline-like tissues.

Two- and three-dimensional in vitro nucleus pulposus cultures: An in silico analysis of local nutrient microenvironments

Background

It is well established that the unique biochemical microenvironment of the intervertebral disc plays a predominant role in cell viability and biosynthesis. However, unless the effect of microenvironmental conditions is primary to a study objective, in vitro culture parameters that are critical for reproducibility are both varied and not routinely reported.

Aims

This work aims to investigate the local microenvironments of commonly used culture configurations, highlighting physiological relevance, potential discrepancies, and elucidating possible heterogeneity across the research field.

Materials and Methods

This work uses nutrient-transport in silico models to reflect on the effect of often underappreciated parameters, such as culture geometry and diffusional distance (vessel, media volume, construct size), seeding density, and external boundary conditions on the local microenvironment of two-dimensional (2D) and three-dimensional (3D) in vitro culture systems.

Results

We elucidate important discrepancies between the external boundary conditions such as the incubator level or media concentrations and the actual local cellular concentrations. Oxygen concentration and cell seeding density were found to be highly influential parameters and require utmost consideration when utilizing 3D culture systems.

Discussion

This work highlights that large variations in the local nutrient microenvironment can easily be established without consideration of several key parameters. Without careful deliberation of the microenvironment within each specific and unique system, there is the potential to confound in vitro results leading to heterogeneous results across the research field in terms of biosynthesis and matrix composition.

Conclusion

Overall, this calls for a greater appreciation of key parameters when designing in vitro experiments. Better harmony and standardization of physiologically relevant local microenvironments are needed to push toward reproducibility and successful translation of findings across the research field.

From Tissue Physoxia to Cancer Hypoxia, Cost-Effective Methods to Study Tissue-Specific O2 Levels in Cellular Biology

The human body is endowed with an extraordinary ability to maintain different oxygen levels in various tissues and organs. The maintenance of physiological levels of oxygen is known as physoxia. The development of hypoxic conditions plays an important role in the biology of several pathologies, including cancer. In vitro studies using normal and neoplastic cells require that culture conditions be carried out under appropriate oxygen levels, either physoxic or hypoxic conditions. Such requirements are difficult to widely implement in laboratory practice, mainly due to the high costs of specialized equipment. In this work, we present and characterize a cost-effective method to culture cells under a range of oxygen levels using deoxidizing pouches. Our results show that physoxic and hypoxic levels using deoxidizing absorbers can be achieved either by implementing a gradual change in oxygen levels or by a regimen of acute depletion of oxygen. This approach triggers the activation of an epithelial-mesenchymal transition in cancer cells while stimulating the expression of HIF-1α. Culturing cancer cells with deoxidizing agent pouches revealed PI3K oncogenic pathway exacerbations compared to tumor cells growing under atmospheric levels of oxygen. Similar to the PI3K signaling disturbance, we also observed augmented oxidative stress and superoxide levels and increased cell cycle arrest. Most interestingly, the culture of cancer cells under hypoxia resulted in the accumulation of cancer stem cells in a time-dependent manner. Overall, we present an attractive, cost-effective method of culturing cells under appropriate physoxic or hypoxic conditions that is easily implementable in any wet laboratory equipped with cell culture tools.

Electrochemically induced in vitro focal hypoxia in human neurons

Focalised hypoxia is widely prevalent in diseases such as stroke, cardiac arrest, and dementia. While in some cases hypoxia improves cellular functions, it mostly induces or exacerbates pathological changes. The lack of methodologies that can simulate focal acute hypoxia, in either animal or cell culture, impedes our understanding of the cellular consequences of hypoxia. To address this gap, an electrochemical localised oxygen scavenging system (eLOS), is reported, providing an innovative platform for spatiotemporal in vitro hypoxia modulation. The electrochemical system is modelled showing O₂ flux patterns and localised O₂ scavenging and hypoxia regions, as a function of distance from the electrode and surrounding flux barriers, allowing an effective focal hypoxia tool to be designed for in vitro cell culture study. O₂ concentration is reduced in an electrochemically defined targeted area from normoxia to hypoxia in about 6 min depending on the O₂-flux boundaries. As a result, a cell culture-well was designed, where localised O₂ scavenging could be induced. The impact of localised hypoxia was demonstrated on human neural progenitor cells (hNPCs) and it was shown that miniature focal hypoxic insults can be induced, that evoke time-dependent HIF-1α transcription factor accumulation. This transcription is "patterned" across the culture according to the electrochemically induced spatiotemporal hypoxia gradient. A basic lacunar infarct model was also developed through the application of eLOS in a purpose designed microfluidic device. Miniature focal hypoxic insults were induced in cellular processes of fully oxygenated cell bodies, such as the axons of human cortical neurons. The results demonstrate experimentally that localised axonal hypoxic stress can lead to significant increase of neuronal death, despite the neurons remaining at normoxia. This suggests that focal hypoxic insult to axons alone is sufficient to impact surrounding neurons and may provide an in vitro model to study the impact of microinfarcts occurring in the deep cerebral white matter, as well as providing a promising tool for wider understanding of acute hypoxic insults with potential to uncover its pathophysiology in multiple diseases.

Acute vs. chronic vs. intermittent hypoxia in breast Cancer: a review on its application in in vitro research

Hypoxia has been linked to elevated instances of therapeutic resistance in breast cancer. The exposure of proliferating cancer cells to hypoxia has been shown to induce an aggressive phenotype conducive to invasion and metastasis. Regions of the primary tumors in the breast may be exposed to different types of hypoxia including acute, chronic or intermittent. Intermittent hypoxia (IH), also called cyclic hypoxia, is caused by exposure to cycles of hypoxia and reoxygenation (H-R cycles). Importantly, there is currently no consensus amongst the scientific community on the total duration of hypoxia, the oxygen level, and the possible presence of H-R cycles. In this review, we discuss current methods of hypoxia research, to explore how exposure regimes used in experiments are connected to signaling by different hypoxia inducible factors (HIFs) and to distinct cellular responses in the context of the hallmarks of cancer. We highlight discrepancies in the existing literature on hypoxia research within the field of breast cancer in particular and propose a clear definition of acute, chronic, and intermittent hypoxia based on HIF activation and cellular responses: (i) acute hypoxia is when the cells are exposed for no more than 24 h to an environment with 1% O₂ or less; (ii) chronic hypoxia is when the cells are exposed for more than 48 h to an environment with 1% O₂ or less and (iii) intermittent hypoxia is when the cells are exposed to at least two rounds of hypoxia (1% O₂ or less) separated by at least one period of reoxygenation by exposure to normoxia (8.5% O₂ or higher). Our review provides for the first time a guideline for definition of hypoxia related terms and a clear foundation for hypoxia related in vitro (breast) cancer research.

Practical Considerations of Dissolved Oxygen Levels for Platelet Function under Hypoxia

Investigating human platelet function in low-oxygen environments is important in multiple settings, including hypobaric hypoxia (e.g., high altitude), sea level hypoxia-related disease, and thrombus stability. These studies often involve drawing blood from which platelets are isolated and analysed at atmospheric conditions or re-exposed to low oxygen levels in hypoxia chambers before testing. However, it remains unknown how the in vitro handling of the samples itself changes their dissolved oxygen concentration, which might affect platelet function and experimental results. Here, we prepared healthy donor platelet-rich plasma and washed platelet (WP) suspensions and exposed them to 2% oxygen. We found that the use of hypoxia pre-equilibrated tubes, higher platelet concentrations (>2 × 10⁸/mL versus 2 × 10⁷/mL), smaller volumes (600 µL versus 3 mL), and presence of plasma reduced the time for samples to reach 2% oxygen. Notably, oxygen levels decreased below 2% in most suspensions, but also in WP maintained at atmospheric 21% oxygen. Additionally, platelet spreading on fibrinogen was decreased when using hypoxic fibrinogen-coated culture plates regardless of the oxygen percentage (2% or 21%) in which platelet incubation took place. Thus, sample handling and experimental conditions should be carefully monitored in platelet-hypoxia studies as they might compromise results interpretation and comparison across studies.

Hyperbaric oxygen therapy induces transcriptome changes in elderly: a prospective trial

Introduction: Aging is characterized by the progressive loss of physiological capacity. Changes in gene expression can alter activity in defined age-related molecular pathways leading to cellular aging and increased aging disease susceptibility. The aim of the current study was to evaluate whether hyperbaric oxygen therapy (HBOT) affects gene expression in normal, non-pathological, aging adults.

Methods: Thirty-five healthy independently living adults, aged 64 and older, were enrolled to receive 60 daily HBOT exposures. Whole blood samples were collected at baseline, at the 30th and 60th HBOT session, and 1–2 weeks following the last session. Differential gene expression analysis was performed.

Results: Following 60 sessions of HBOT, 1342 genes and 570 genes were differently up- and downregulated (1912 total), respectively (p < 0.01 FDR), compared to baseline. Out of which, five genes were downregulated with a >1.5-fold change: ABCA13 (FC = −2.28), DNAJ6 (FC = −2.16), HBG2 (FC = −1.56), PDXDC1 (FC = −1.53), RANBP17 (FC = −1.75). Two weeks post-HBOT, ABCA13 expression was significantly downregulated with a >1.5fold change (FC = −1.54, p = 0.008).

In conclusion, for the first time in humans, the study provides direct evidence of HBOT is associated with transcriptome changes in whole-blood samples. Our results demonstrate significant changes in gene expression of normal aging population.

Engineering Tools for Regulating Hypoxia in Tumour Models

Major advances in the field of genomic technologies have led to an improvement in cancer diagnosis, classification and prognostication. However, many cancers remain incurable due to the development of drug resistance, minimal residual disease (MRD) and disease relapse, highlighting an incomplete understanding of the mechanisms underlying these processes. In recent years, the impact of non-genetic factors on neoplastic transformations has increasingly been acknowledged, and growing evidence suggests that low oxygen (O2 ) levels (ie hypoxia) in the tumour microenvironment play a critical role in the development and treatment of cancer. As a result, there is a growing need to develop research tools capable of reproducing physiologically relevant O2 conditions encountered by cancer cells in their natural environments in order to gain in-depth insight into tumour cell metabolism and function. In this review, the authors highlight the importance of hypoxia in the pathogenesis of malignant diseases and provide an overview of novel engineering tools that have the potential to further drive this evolving, yet technically challenging, field of cancer research.

Sorafenib prevents the proliferation and induces the apoptosis of liver cancer cells by regulating autophagy and hypoxia-inducible factor-1

Sorafenib has been approved as a systemic drug for advanced liver cancer; however, the underlying mechanisms remain unclear. The present study aimed to investigate the effects of sorafenib on the proliferation, autophagy and apoptosis of HepG2 cells under hypoxia. Briefly, reverse transcription-quantitative PCR and western blotting was performed to quantify HIF-1, LC3II/I, mTOR and p70s6K expression levels. Cell proliferation was determined using the Cell Counting Kit-8 assay and the cell apoptosis rate was evaluated using flow cytometry. The results demonstrated that autophagy and apoptosis were induced by hypoxia, and that sorafenib further enhanced hypoxia-induced autophagy and apoptosis in HepG2 cells in a dose-dependent manner. Furthermore, the mechanism of sorafenib-mediated autophagy in liver cancer cell were investigated by using chloroquine (CQ). The results showed that CQ significantly inhibited autophagy by decreasing LC3II/LC3I ratio in HepG2 cells treated with sorafenib and/or hypoxia. By contrast, sorafenib could increase the expression of hypoxia-inducible factor-1 (HIF-1) and of the autophagy marker (LC3II/I) and decrease the expression of mammalian target of rapamycin and p70 ribosomal S6 kinase in HepG2 cells under normoxia and hypoxia conditions, suggesting that sorafenib could induce hypoxia and autophagy in liver cancer cells. In addition, sorafenib was demonstrated to prevent proliferation and induce apoptosis of HepG2 cells under normoxia and hypoxia. Sorafenib could also prevent the malignant behavior of HepG2 by inducing hypoxia and autophagy. In summary, the findings from the present study suggested that sorafenib may inhibit liver cancer progression by activating autophagy and HIF-1 signaling pathway.

Considering Cell Proliferation to Optimize Detection of Radiation-Induced 53BP1 Positive Foci in 15 Mouse Strains Ex Vivo

Due to high metabolic activity, proliferating cells continuously generate free radicals, which induce DNA double-strand breaks (DSB). Fluorescently tagged nuclear foci of DNA repair protein 53 binding protein-1 (53BP1) are used as a standard metric for measuring DSB formation at baseline and in response to environmental insults such as radiation. Here we demonstrate that the background level of spontaneous 53BP1+ foci formation can be modeled mathematically as a function of cell confluence, which is a metric of their proliferation rate. This model was validated using spontaneous 53BP1+ foci data from 72 cultures of primary skin fibroblasts derived from 15 different strains of mice, showing a ∼10-fold decrease from low to full confluence that is independent of mouse strain. On the other hand, the baseline level of spontaneous 53BP1+ foci in a fully confluent cell population was strain-dependent, suggesting genomic associations, and correlated with radiation sensitivity based on previous measurements in the same cell lines. Finally, we have developed an online open-access tool to correct for the effect of cell confluence on 53BP1+ foci-based quantification of DSB. This tool provides guidelines for the number of cells required to reach statistical significance for the detection of DSB induced by low doses of ionizing radiation as a function of confluence and time postirradiation.

A Microfluidic Chip Architecture Enabling a Hypoxic Microenvironment and Nitric Oxide Delivery in Cell Culture

A hypoxic (low oxygen level) microenvironment and nitric oxide paracrine signaling play important roles in the control of both biological and pathological cell responses. In this study, we present a microfluidic chip architecture for nitric oxide delivery under a hypoxic microenvironment in human embryonic kidney cells (HEK-293). The chip utilizes two separate, but interdigitated microfluidic channels. The hypoxic microenvironment was created by sodium sulfite as the oxygen scavenger in one of the channels. The nitric oxide microenvironment was created by sodium nitroprusside as the light-activated nitric oxide donor in the other channel. The solutions are separated from the cell culture by a 30 µm thick gas-permeable, but liquid-impermeable polydimethylsiloxane membrane. We show that the architecture is preliminarily feasible to define the gaseous microenvironment of a cell culture in the 100 µm and 1 mm length scales.

Simulation of hypoxia of myocardial cells in microfluidic systems

The paper presents a newly designed microfluidic system that allows simulation of myocardial hypoxia by biochemical method. The geometry of the microsystem was designed in such a way, that quantitative fluorescent measurements using a spectrofluorometric plate reader was possible. Biochemical simulation of hypoxia was carried out using potent mitochondrial oxidative phosphorylation uncoupler—Carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone (FCCP). Two cardiac cell lines were used in the study—rat cardiomyoblasts (H9C2) and human cardiomyocytes. The effectiveness of biochemical simulation of hypoxia was studied using two fluorescent dyes: carbocyanine iodide (JC-1) and Fluo-4. Changes in the mitochondrial membrane potential and concentration of intracellular calcium ions were tested. The major novelty of this research was the applying the microfluidic system to create hypoxia conditions for cardiac cells using the biochemical approach. In further studies, the presented hypoxia model could be used to develop new methods of treatment of ischemic heart disease for example in cell therapy based on stem cells.

To breathe or not to breathe? Hypoxia after pulsed-electric field treatment reduces the effectiveness of electrochemotherapy in vitro

Bleomycin, which is the most widely used drugs in electrochemotherapy, requires oxygen to be able to make single- or double-strand brakes in DNA. However, the concentration of oxygen in tumours can be lower than 1%. The aim of this study was to find out whether oxygen concentration in the medium in which cells loaded with bleomycin are incubated, affects the effectiveness of electrochemotherapy in vitro. Experiments were carried out on mouse hepatoma MH-22A cells. Cells were loaded with bleomycin by using a single square-wave electric pulse (2 kV/cm, 100 μs) under normoxic conditions, seeded into Petri dishes, and grown under normoxic and hypoxic conditions. Cell viability was determined by means of a colony-forming assay. We demonstrated that when cells loaded with bleomycin were incubated in hypoxia (0.2% O₂), up to 5.3-fold higher concentrations of bleomycin were needed to kill them in comparison with cells grown in normoxia (18.7% O₂).

Caveolin-1-Mediated Tumor Suppression Is Linked to Reduced HIF1α S-Nitrosylation and Transcriptional Activity in Hypoxia

Caveolin-1 (CAV1) is a well-established nitric oxide synthase inhibitor, whose function as a tumor suppressor is favored by, but not entirely dependent on, the presence of E-cadherin. Tumors are frequently hypoxic and the activation of the hypoxia-inducible factor-1α (HIF1α) promotes tumor growth. HIF1α is regulated by several post-translational modifications, including S-nitrosylation. Here, we evaluate the mechanisms underlying tumor suppression by CAV1 in cancer cells lacking E-cadherin in hypoxia. Our main findings are that CAV1 reduced HIF activity and Vascular Endothelial Growth Factor expression in vitro and in vivo. This effect was neither due to reduced HIF1α protein stability or reduced nuclear translocation. Instead, HIF1α S-nitrosylation observed in hypoxia was diminished by the presence of CAV1, and nitric oxide synthase (NOS) inhibition by Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME) reduced HIF1α transcriptional activity in cells to the same extent as observed upon CAV1 expression. Additionally, arginase inhibition by (S)-(2-Boronoethyl)-L-cysteine (BEC) partially rescued cells from the CAV1-mediated suppression of HIF1α transcriptional activity. In vivo, CAV1-mediated tumor suppression was dependent on NOS activity. In summary, CAV1-dependent tumor suppression in the absence of E-cadherin is linked to reduced HIF1α transcriptional activity via diminished NOS-mediated HIF1α S-nitrosylation.

The Role of Hypoxia-Inducible Factor 1α in the Regulation of Human Meibomian Gland Epithelial Cells

Purpose

We recently discovered that a hypoxic environment is beneficial for meibomian gland (MG) function. The mechanisms underlying this effect are unknown, but we hypothesize that it is due to an increase in the levels of hypoxia-inducible factor 1α (HIF1α). In other tissues, HIF1α is the primary regulator of cellular responses to hypoxia, and HIF1α expression can be induced by multiple stimuli, including hypoxia and hypoxia-mimetic agents. The objective of this study was to test our hypothesis.

Methods

Human eyelid tissues were stained for HIF1α. Immortalized human MG epithelial cells (IHMGECs) were cultured for varying time periods under normoxic (21% O2) or hypoxic (1% O2) conditions, in the presence or absence of the hypoxia-mimetic agent roxadustat (Roxa). IHMGECs were then processed for the analysis of cell number, HIF1α expression, lipid-containing vesicles, neutral and polar lipid content, DNase II activity, and intracellular pH.

Results

Our results show that HIF1α protein is present in human MG acinar epithelial cells in vivo. Our findings also demonstrate that exposure to 1% O2 or to Roxa increases the expression of HIF1α, the number of lipid-containing vesicles, the content of neutral lipids, and the activity of DNase II and decreases the pH in IHMGECs in vitro.

Conclusions

Our data support our hypothesis that the beneficial effect of hypoxia on the MG is mediated through an increased expression of HIF1α.

A New Non-invasive Technique for Measuring 3D-Oxygen Gradients in Wells During Mammalian Cell Culture

Oxygen tension plays an important role in overall cell function and fate, regulating gene expression, and cell differentiation. Although there is extensive literature available that supports the previous statement, little information is to be found about accurate O2 measurements during culture. In fact, O2 concentration at the cell layer during culture is commonly assumed to be equal to that of the incubator atmosphere. This assumption does not consider oxygen diffusion properties, cell type, cell density, media composition, time in culture nor height of the cell culture medium column. In this study, we developed a non-invasive, optical sensor foil-based technique suitable for measuring the 3D oxygen gradient that is formed during cell culture as a result of normal cell respiration. For this propose, we created a 3D printed ramp to which surface an oxygen optode sensor foil was attached. The ramps were positioned inside the culture wells of 24 well plate prior cell seeding. This set up in conjunction with the VisiSens TD camera system allows to investigate the oxygen gradient formation during culture. Cultivation was performed with three different initial cell densities of the cell line A549 that were seeded on the plate containing the ramps with the oxygen sensors. The O2 gradient obtained after 96 h of culture showed significantly lower O2 concentrations closer to the bottom of the well in high cell density cultures compared to that of lower cell density cultures. Furthermore, it was very interesting to observe that even with low cell density culture, oxygen concentration near the cell layer was lower than that of the incubator atmosphere. The obtained oxygen gradient after 96 h was used to calculate the oxygen consumption rate (OCR) of the A549 cells, and the obtained value of ~100 fmol/h/cell matches the OCR value already reported in the literature for this cell line. Moreover, we found our set up to be unique in its ability to measure oxygen gradient formation in several wells of a cell culture plate simultaneously and in a non-invasive manner.

Modelling human CNS injury with human neural stem cells in 2- and 3-Dimensional cultures

The adult human central nervous system (CNS) has very limited regenerative capability, and injury at the cellular and molecular level cannot be studied in vivo. Modelling neural damage in human systems is crucial to identifying species-specific responses to injury and potentially neurotoxic compounds leading to development of more effective neuroprotective agents. Hence we developed human neural stem cell (hNSC) 3-dimensional (3D) cultures and tested their potential for modelling neural insults, including hypoxic-ischaemic and Ca2+-dependent injury. Standard 3D conditions for rodent cells support neuroblastoma lines used as human CNS models, but not hNSCs, but in all cases changes in culture architecture alter gene expression. Importantly, response to damage differs in 2D and 3D cultures and this is not due to reduced drug accessibility. Together, this study highlights the impact of culture cytoarchitecture on hNSC phenotype and damage response, indicating that 3D models may be better predictors of in vivo response to damage and compound toxicity.

Technical Feasibility and Physiological Relevance of Hypoxic Cell Culture Models

Hypoxia is characterized as insufficient oxygen delivery to tissues and cells in the body and is prevalent in many human physiology processes and diseases. Thus, it is an attractive state to experimentally study to understand its inner mechanisms as well as to develop and test therapies against pathological conditions related to hypoxia. Animal models in vivo fail to recapitulate some of the key hallmarks of human physiology, which leads to human cell cultures; however, they are prone to bias, namely when pericellular oxygen concentration (partial pressure) does not respect oxygen dynamics in vivo. A search of the current literature on the topic revealed this was the case for many original studies pertaining to experimental models of hypoxia in vitro. Therefore, in this review, we present evidence mandating for the close control of oxygen levels in cell culture models of hypoxia. First, we discuss the basic physical laws required for understanding the oxygen dynamics in vitro, most notably the limited diffusion through a liquid medium that hampers the oxygenation of cells in conventional cultures. We then summarize up-to-date knowledge of techniques that help standardize the culture environment in a replicable fashion by increasing oxygen delivery to the cells and measuring pericellular levels. We also discuss how these tools may be applied to model both constant and intermittent hypoxia in a physiologically relevant manner, considering known values of partial pressure of tissue normoxia and hypoxia in vivo, compared to conventional cultures incubated at rigid oxygen pressure. Attention is given to the potential influence of three-dimensional tissue cultures and hypercapnia management on these models. Finally, we discuss the implications of these concepts for cell cultures, which try to emulate tissue normoxia, and conclude that the maintenance of precise oxygen levels is important in any cell culture setting.

Intermittent Hypoxia Stimulates Lipolysis, But Inhibits Differentiation and De Novo Lipogenesis in 3T3-L1 Cells

Background: Exposure to intermittent hypoxia (IH) may play a role in the development of metabolic impairments in the context of obstructive sleep apnea syndrome, probably by elevated plasma levels of free fatty acids. Employing gas-permeable cultureware to grow differentiated human and mouse adipocytes in vitro, we directly studied the effects of pericellular oxygen fluctuations on key adipocyte metabolic functions-spontaneous lipolytic rates, triglyceride accumulation, de novo lipogenesis, and expression of adipocyte-specific marker genes. Materials and Methods: 3T3-L1 fibroblasts and human subcutaneous preadipocytes were differentiated under conditions that induced repetitive pericellular-oxygen cycles IH between 1% O₂ (5 min) and 16% O₂ (5 min), continuously for 14 days or under control conditions. Chemicals were used to inhibit the flux of acetyl-CoA from glycolysis (alfa-cyano-4-hydroxy cinnamate) or the tricarboxylic acid cycle (SB204990), or to stimulate the flux of acetyl-CoA from pyruvate to the lipogenic pool. Lipolytic rate, intracellular lipids, and expression of adipocyte differentiation markers were assessed and t-test or ANOVA were used to find significant differences. Results: The rate of lipolysis increased by 211% in 3T3-L1 cells and by 39% in obese human adipocytes. Exposure to IH reduced intracellular lipid stores by 37% and reduced the expression of adipocyte differentiation markers. Pharmacological stimulation or inhibition of de novo lipogenesis did not modify the intracellular lipid content under IH. Conclusions: Pericellular oxygen fluctuations directly stimulated lipolysis, but did not increase de novo lipogenesis from endogenous substrates. Similarly, IH hampered adipocyte differentiation from precursors.

A Cell Culture Model that Mimics Physiological Tissue Oxygenation Using Oxygen-permeable Membranes

Dissolved oxygen and its availability to cells in culture is an overlooked variable which can have significant consequences on experimental research outcomes, including reproducibility. Oxygen sensing pathways play key roles in cell growth and behavior and pericellular oxygen levels should be controlled when establishing in vitro models. Standard cell culture techniques do not have adequate control over pericellular oxygen levels. Slow diffusion through culture media limits the precision of oxygen delivery to cells, making it difficult to accurately reproduce in vivo-like oxygen conditions. Furthermore, different types of cells consume oxygen at varying rates and this can be affected by the density of growing cells. Here, we describe a novel in vitro system that utilizes hypoxic chambers and oxygen-permeable culture dishes to control pericellular oxygen levels and provide rapid oxygen delivery to adherent cells. This procedure is particularly relevant for protocols studying effects of rapid oxygen changes or intermittent hypoxia on cellular behavior. The system is inexpensive and easily assembled without highly specialized equipment.

Chronic hypoxia changes gene expression profile of primary rat carotid body cells: consequences on the expression of NOS isoforms and ET-1 receptors

Sustained chronic hypoxia (CH) produces morphological and functional changes in the carotid body (CB). Nitric oxide (NO) and endothelin-1 (ET-1) play a major role as modulators of the CB oxygen chemosensory process. To characterize the effects of CH related to normoxia (Nx) on gene expression, particularly on ET-1 and NO pathways, primary cultures of rat CB cells were exposed to 7 days of CH. Total RNA was extracted, and cDNA-32P was synthesized and hybridized with 1,185 genes printed on a nylon membrane Atlas cDNA Expression Array. Out of 324 differentially expressed genes, 184 genes were upregulated, while 140 genes were downregulated. The cluster annotation and protein network analyses showed that both NO and ET-1 signaling pathways were significantly enriched and key elements of each pathway were differentially expressed. Thus, we assessed the effect of CH at the protein level of nitric oxide synthase (NOS) isoforms and ET-1 receptors. CH induced an increase in the expression of endothelial NOS, inducible NOS, and ETB. During CH, the administration of SNAP, a NO donor, upregulated ETB. Treatment with Tezosentan (ET-1 receptor blocker) during CH upregulated all three NOS isoforms, while the NOS blocker L-NAME induced upregulation of iNOS and ETB and downregulated the protein levels of ETA. These results show that CH for 7 days changed the cultured cell CB gene expression profile, the NO and ET-1 signaling pathways were highly enriched, and these two signaling pathways interfered with the protein expression of each other.

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Optimizing the Cell Culture Microenvironment

The survival, proliferation, and differentiation of cells in culture are determined not only by their intrinsic potential but also by cues provided by the permissive or restrictive microenvironment in which they reside. The robustness and reproducibility of cell culture assays and endpoints relies on the stability of that microenvironment and vigilant attention to the control of variables that affect cell behavior during culture. These often underappreciated variables include, but are not limited to, medium pH and buffering, osmolarity, composition of the gas phase, the timing and periodicity of refeeding and subculture, and the impact of fluctuations in temperature and gas phase composition on frequent opening and closing of incubator doors. This chapter briefly describes the impact of these and other variables on the behavior of cultured cells.

Molecular Mechanism of Ischaemic Preconditioning of Skeletal Muscle In Vitro

Introduction

Ischaemic preconditioning (IPC) is a phenomenon whereby tissues develop an increased tolerance to ischaemia and subsequent reperfusion if first subjected to sublethal periods of ischaemia. Despite extensive investigation of IPC, the molecular mechanism remains largely unknown. Our aim was to show genetic changes that occur in skeletal muscle cells in response to IPC.

Methods

We established an in vitro model of IPC using a human skeletal muscle cell line. Gene expression of both control and preconditioned cells at various time points was determined. The genes examined were hypoxia-inducible factor-1 alpha (HIF-1 alpha), early growth response 1 (EGR1), JUN, and FOS. HIF-1 alpha is a marker of hypoxia. EGR1, JUN, and FOS are early response genes and may play a role in the protective responses induced by IPC.

Results

HIF-1 alpha was upregulated following one and two hours of simulated ischaemia (p = 0.076 and 0.841, respectively) verifying that hypoxic conditions were met using our model. Expression of EGR1 and FOS was upregulated and peaked after one hour of hypoxia (p = 0.001 and <0.00, respectively). cFOS was upregulated at two and three hours of hypoxia. IPC prior to simulated hypoxia resulted in a greater level of upregulation of EGR1, JUN and FOS genes (p = <0.00, 0.047, and <0.00 respectively).

Conclusion

This study has supported the use of our hypoxic model for studying IPC in vitro. IPC results in a greater upregulation of protective genes in skeletal muscle cells exposed to hypoxia than in control cells. We have demonstrated hitherto unknown molecular mechanisms of IPC in cell culture.

Gas Partial Pressure in Cultured Cells: Patho-Physiological Importance and Methodological Approaches

Gas partial pressures within the cell microenvironment are one of the key modulators of cell pathophysiology. Indeed, respiratory gases (O2 and CO2) are usually altered in respiratory diseases and gasotransmitters (CO, NO, H2S) have been proposed as potential therapeutic agents. Investigating the pathophysiology of respiratory diseases in vitro mandates that cultured cells are subjected to gas partial pressures similar to those experienced by each cell type in its native microenvironment. For instance, O2 partial pressures range from ∼13% in the arterial endothelium to values as low as 2-5% in cells of other healthy tissues and to less than 1% in solid tumor cells, clearly much lower values than those used in conventional cell culture research settings (∼19%). Moreover, actual cell O2 partial pressure in vivo changes with time, at considerably different timescales as illustrated by tumors, sleep apnea, or mechanical ventilation. Unfortunately, the conventional approach to modify gas concentrations at the above culture medium precludes the tight and exact control of intra-cellular gas levels to realistically mimic the natural cell microenvironment. Interestingly, well-controlled cellular application of gas partial pressures is currently possible through commercially available silicone-like material (PDMS) membranes, which are biocompatible and have a high permeability to gases. Cells are seeded on one side of the membrane and tailored gas concentrations are circulated on the other side of the membrane. Using thin membranes (50-100 μm) the value of gas concentration is instantaneously (<0.5 s) transmitted to the cell microenvironment. As PDMS is transparent, cells can be concurrently observed by conventional or advanced microscopy. This procedure can be implemented in specific-purpose microfluidic devices and in settings that do not require expensive or complex technologies, thus making the procedure readily implementable in any cell biology laboratory. This review describes the gas composition requirements for a cell culture in respiratory research, the limitations of current experimental settings, and also suggests new approaches to better control gas partial pressures in a cell culture.

Transportable system enabling multiple irradiation studies under simultaneous hypoxia in vitro

Background

Cells in solid tumours are variably hypoxic and hence resistant to radiotherapy - the essential role of oxygen in the efficiency of irradiation has been acknowledged for decades. However, the currently available methods for performing hypoxic experiments in vitro have several limitations, such as a limited amount of parallel experiments, incapability of keeping stable growth conditions and dependence on CO₂ incubator or a hypoxia workstation. The purpose of this study was to evaluate the usability of a novel portable system (Minihypoxy) in performing in vitro irradiation studies under hypoxia, and present supporting biological data.

Materials and methods

This study was conducted on cancer cell cultures in vitro. The cells were cultured in normoxic (~ 21% O₂) or in hypoxic (1% O₂) conditions either in conventional hypoxia workstation or in the Minihypoxy system and irradiated at dose rate 1.28 Gy/min ± 2.9%. The control samples were sham irradiated. To study the effects of hypoxia and irradiation on cell viability and DNA damage, western blotting, immunostainings and clonogenic assay were used. The oxygen level, pH, evaporation rate and osmolarity of the culturing media on cell cultures in different conditions were followed.

Results

The oxygen concentration in interest (5, 1 or 0% O₂) was maintained inside the individual culturing chambers of the Minihypoxy system also during the irradiation. The radiosensitivity of the cells cultured in Minihypoxy chambers was declined measured as lower phosphorylation rate of H2A.X and increased clonogenic capacity compared to controls (OER~ 3).

Conclusions

The Minihypoxy system allows continuous control of hypoxic environment in multiple wells and is transportable. Furthermore, the system maintains the low oxygen environment inside the individual culturing chambers during the transportation and irradiation in experiments which are typically conducted in separate facilities.

Electronic supplementary material

The online version of this article (10.1186/s13014-018-1169-9) contains supplementary material, which is available to authorized users.

Mixing and delivery of multiple controlled oxygen environments to a single multiwell culture plate

Precise oxygen control is critical to evaluating cell growth, molecular content, and stress response in cultured cells. We have designed, fabricated, and characterized a 96-well plate-based device that is capable of delivering eight static or dynamically changing oxygen environments to different rows on a single plate. The device incorporates a gas-mixing tree that combines two input gases to generate the eight gas mixtures that supply each row of the plate with a different gas atmosphere via a removable manifold. Using air and nitrogen as feed gases, a single 96-well plate can culture cells in applied gas atmospheres with Po₂ levels ranging from 1 to 135 mmHg. Human cancer cell lines MCF-7, PANC-1, and Caco-2 were grown on a single plate under this range of oxygen levels. Only cells grown in wells exposed to Po₂ ≤37 mmHg express the endogenous hypoxia markers hypoxia-inducible factor-1α and carbonic anhydrase IX. This design is amenable to multiwell plate-based molecular assays or drug dose-response studies in static or cycling hypoxia conditions.

Oxygenation in cell culture: Critical parameters for reproducibility are routinely not reported

Mammalian cell culture is foundational to biomedical research, and the reproducibility of research findings across the sciences is drawing increasing attention. While many components contribute to reproducibility, the reporting of factors that impact oxygen delivery in the general biomedical literature has the potential for both significant impact, and immediate improvement. The relationship between the oxygen consumption rate of cells and the diffusive delivery of oxygen through the overlying medium layer means parameters such as medium depth and cell type can cause significant differences in oxygenation for cultures nominally maintained under the same conditions. While oxygenation levels are widely understood to significantly impact the phenotype of cultured cells in the abstract, in practise the importance of the above parameters does not appear to be well recognized in the non-specialist research community. On analyzing two hundred articles from high-impact journals we find a large majority missing at least one key piece of information necessary to ensure consistency in replication. We propose that explicitly reporting these values should be a requirement for publication.

Development of an In Vitro Cardiac Ischemic Model Using Primary Human Cardiomyocytes

Developing experimental models to study ischemic heart disease is necessary for understanding of biological mechanisms to improve the therapeutic approaches for restoring cardiomyocytes function following injury. The aim of this study was to develop an in vitro hypoxic/re-oxygenation model of ischemia using primary human cardiomyocytes (HCM) and define subsequent cytotoxic effects. HCM were cultured in serum and glucose free medium in hypoxic condition with 1% O₂ ranging from 30 min to 12 h. The optimal hypoxic exposure time was determined using Hypoxia Inducible Factor 1α (HIF-1α) as the hypoxic marker. Subsequently, the cells were moved to normoxic condition for 3, 6 and 9 h to replicate the re-oxygenation phase. Optimal period of hypoxic/re-oxygenation was determined based on 50% mitochondrial injury via 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide assay and cytotoxicity via lactate dehydrogenase (LDH) assay. It was found that the number of cells expressing HIF-1α increased with hypoxic time and 3 h was sufficient to stimulate the expression of this marker in all the cells. Upon re-oxygenation, mitochondrial activity reduced significantly whereas the cytotoxicity increased significantly with time. Six hours of re-oxygenation was optimal to induce reversible cell injury. The injury became irreversible after 9 h as indicated by > 60% LDH leakage compared to the control group cultured in normal condition. Under optimized hypoxic reoxygenation experimental conditions, mesenchymal stem cells formed nanotube with ischemic HCM and facilitated transfer of mitochondria suggesting the feasibility of using this as a model system to study molecular mechanisms of myocardial injury and rescue.

Microfluidics in Malignant Glioma Research and Precision Medicine

Glioblastoma multiforme (GBM) is an aggressive form of brain cancer that has no effective treatments and a prognosis of only 12-15 months. Microfluidic technologies deliver microscale control of fluids and cells, and have aided cancer therapy as point-of-care devices for the diagnosis of breast and prostate cancers. However, a few microfluidic devices are developed to study malignant glioma. The ability of these platforms to accurately replicate the complex microenvironmental and extracellular conditions prevailing in the brain and facilitate the measurement of biological phenomena with high resolution and in a high-throughput manner could prove useful for studying glioma progression. These attributes, coupled with their relatively simple fabrication process, make them attractive for use as point-of-care diagnostic devices for detection and treatment of GBM. Here, the current issues that plague GBM research and treatment, as well as the current state of the art in glioma detection and therapy, are reviewed. Finally, opportunities are identified for implementing microfluidic technologies into research and diagnostics to facilitate the rapid detection and better therapeutic targeting of GBM.

Live Cell Imaging in Microfluidic Device Proves Resistance to Oxygen/Glucose Deprivation in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Analyses of cellular responses to fast oxygen dynamics are challenging and require ad hoc technological solutions, especially when decoupling from liquid media composition is required. In this work, we present a microfluidic device specifically designed for culture analyses with high resolution and magnification objectives, providing full optical access to the cell culture chamber. This feature allows fluorescence-based assays, photoactivated surface chemistry, and live cell imaging under tightly controlled pO₂ environments. The device has a simple design, accommodates three independent cell cultures, and can be employed by users with basic cell culture training in studies requiring fast oxygen dynamics, defined media composition, and in-line data acquisition with optical molecular probes. We apply this technology to produce an oxygen/glucose deprived (OGD) environment and analyze cell mortality in murine and human cardiac cultures. Neonatal rat ventricular cardiomyocytes show an OGD time-dependent sensitivity, resulting in a robust and reproducible 66 ± 5% death rate after 3 h of stress. Applying an equivalent stress to human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) provides direct experimental evidence for fetal-like OGD-resistant phenotype. Investigation on the nature of such phenotype exposed large glycogen deposits. We propose a culture strategy aimed at depleting these intracellular energy stores and concurrently activate positive regulation of aerobic metabolic molecular markers. The observed process, however, is not sufficient to induce an OGD-sensitive phenotype in hiPS-CMs, highlighting defective development of mature aerobic metabolism in vitro.

Low-Frequency Intermittent Hypoxia Promotes Subcutaneous Adipogenic Differentiation

Obstructive sleep apnea (OSA), characterized by intermittent hypoxia (IH), is associated with obesity and metabolic disorders. The mass and function of adipose tissue are largely dependent on adipogenesis. The impact of low-frequency IH on adipogenesis is unknown. Sprague-Dawley rats were subjected to IH (4 min for 10% O₂ and 2 min for 21% O₂) or intermittent normoxia (IN) for 6 weeks. The degree of adipogenic differentiation was evaluated by adipogenic transcriptional factors, adipocyte-specific proteins, and oily droplet production in both subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT). Upregulation of proadipogenic markers (CEBPα, PPARγ, and FABP4) and downregulation of antiadipogenic markers CHOP in line with smaller size of adipocytes were found in IH-exposed SAT. In vitro experiments using human preadipocytes (HPAs) of subcutaneous lineage during differentiation phase, subjected to IH (1% O₂ for 10 min and 21% O₂ for 5 min; 5% CO₂) or IN treatment, were done to investigate the insulin-like growth factor 1 receptor (IGF-1R)/Akt pathway in adipogenesis. IH promoted the accumulation of oily droplets and adipogenesis-associated markers. IGF-1R kinase inhibitor NVP-AEW541 attenuated the proadipogenic role in IH-exposed HPAs. In summary, relatively low frequency of IH may enhance adipogenesis preferentially in SAT.

Limitations of oxygen delivery to cells in culture: An underappreciated problem in basic and translational research

Molecular oxygen is one of the most important variables in modern cell culture systems. Fluctuations in its concentration can affect cell growth, differentiation, signaling, and free radical production. In order to maintain culture viability, experimental validity, and reproducibility, it is imperative that oxygen levels be consistently maintained within physiological "normoxic" limits. Use of the term normoxia, however, is not consistent among scientists who experiment in cell culture. It is typically used to describe the atmospheric conditions of a standard incubator, not the true microenvironment to which the cells are exposed. This error may lead to the situation where cells grown in a standard "normoxic" oxygen concentration may actually be experiencing a wide range of conditions ranging from hyperoxia to near-anoxic conditions at the cellular level. This apparent paradox is created by oxygen's sluggish rate of diffusion through aqueous medium, and the generally underappreciated effects that cell density, media volume, and barometric pressure can have on pericellular oxygen concentration in a cell culture system. This review aims to provide an overview of this phenomenon we have termed "consumptive oxygen depletion" (COD), and includes a basic review of the physics, potential consequences, and alternative culture methods currently available to help circumvent this largely unrecognized problem.

Rapid Generation and Detection of Biomimetic Oxygen Concentration Gradients In Vitro

Hypoxic regions exist within most solid tumors and often lead to altered cellular metabolism, metastasis, and drug resistance. Reliable generation and detection of biomimetic gaseous gradients in vitro is challenging due to low spatiotemporal resolution and poor longevity of gradients utilizing microfluidic techniques. Here, we present a novel and simplistic approach for producing gradients of dissolved oxygen (DO) within a lab-on-a-chip platform. Linear and stable DO gradients with high spatial resolution are established by introducing pre-gassed media into the gradient generating network. An underlying platinum(ii) octaethlporphyrin ketone (PtOEPK) based sensor layer allows parallel detection of oxygen. A thin 3-sided glass coating on the inner channel walls prevents multi-directional diffusion of ambient oxygen across PDMS preserving the gradient resolution and stability. Viability analysis of normal mammary epithelial cells (MCF-12A) under oxygen gradients revealed 70% mortality after 6 hours of hypoxic exposure. Biological applicability of the platform was further validated by demonstrating increase in endoplasmic reticulum stress of MDA-MB-468 cells with time and with increasing oxygen tension. The unique ability to establish parallel or opposing gradients of gases and chemicals offers the potential for a wide range of applications in therapeutic development, and fundamental understanding of cellular behavior during hypoxia.

O2-controllable hydrogels for studying cellular responses to hypoxic gradients in three dimensions in vitro and in vivo

Oxygen (O₂) acts as a potent upstream regulator of cell function. In both physiological and pathophysiological microenvironments, the O₂ concentration is not uniformly distributed but instead follows a gradient that depends on distance from oxygen-carrying blood vessels. Such gradients have a particularly important role in development, tissue regeneration, and tumor growth. In this protocol, we describe how to use our previously reported gelatin-based O₂-controllable hydrogels that can provide hypoxic microenvironments in vitro. The hydrogel polymeric network is formed via a laccase-mediated cross-linking reaction. In this reaction, laccase catalyzes diferulic acid (diFA) formation to form hydrogels with an O₂-consuming reaction. Cells, such as cancer or endothelial cells, as well as tumor/tissue grafts, can be encapsulated in the hydrogels during hydrogel formation and then analyzed for cellular responses to 3D hypoxic gradients and to elucidate the underlying mechanisms governing these responses. Importantly, oxygen gradients can be precisely controlled in standard cell/tissue culture conditions and in vivo. This platform has been applied to study vascular morphogenesis in response to hypoxia and to understand how oxygen gradients mediate cancer cell behavior. Herein, we describe the means to validate the assay from polymer synthesis and characterization-which take 1-2 weeks and include verification of ferulic acid (FA) conjugation, rheological measurements, and O₂ monitoring-to the study of cellular responses and use in rodent models. Time courses for biological experiments using this hydrogel are variable, and thus they may range from hours to weeks, depending on the application and user end goal.

Frequency and magnitude of intermittent hypoxia modulate endothelial wound healing in a cell culture model of sleep apnea

Intermittent hypoxia (IH) has been implicated in the cardiovascular consequences of obstructive sleep apnea (OSA). However, the lack of suitable experimental systems has precluded assessment as to whether IH is detrimental, protective, or both for the endothelium. The aim of the work was to determine the effects of frequency and amplitude of IH oxygenation swings on aortic endothelial wound healing. Monolayers of human primary endothelial cells were wounded and subjected to constant oxygenation (1%, 4%, 13%, or 20% O₂) or IH at different frequencies (0.6, 6, or 60 cycles/h) and magnitude ranges (13-4% O₂ or 20-1% O₂), using a novel well-controlled system, with wound healing being measured after 24 h. Cell monolayer repair was similar at 20% O₂ and 13% O₂, but was considerably increased (approximately twofold) in constant hypoxia at 4% O₂ The magnitude and frequency of IH considerably modulated wound healing. Cycles ranging 13-4% O₂ at the lowest frequency (0.6 cycles/h) accelerated endothelial wound healing by 102%. However, for IH exposures consisting of 20% to 1% O₂ oscillations, wound closure was reduced compared with oscillation in the 13-4% range (by 74% and 44% at 6 cycles/h and 0.6 cycles/h, respectively). High-frequency IH patterns simulating severe OSA (60 cycles/h) did not significantly modify endothelial wound closure, regardless of the oxygenation cycle amplitude. In conclusion, the frequency and magnitude of hypoxia cycling in IH markedly alter wound healing responses and emerge as key factors determining how cells will respond in OSA.NEW & NOTEWORTHY Intermittent hypoxia (IH) induces cardiovascular consequences in obstructive sleep apnea (OSA) patients. However, the vast array of frequencies and severities of IH previously employed in OSA-related experimental studies has led to controversial results on the effects of IH. By employing an optimized IH experimental system here, we provide evidence that the frequency and magnitude of IH markedly alter human aortic endothelial wound healing, emerging as key factors determining how cells respond in OSA.

Enzyme-immobilized hydrogels to create hypoxia for in vitro cancer cell culture

Hypoxia is a critical condition governing many aspects of cellular fate processes. The most common practice in hypoxic cell culture is to maintain cells in an incubator with controlled gas inlet (i.e., hypoxic chamber). Here, we describe the design and characterization of enzyme-immobilized hydrogels to create solution hypoxia under ambient conditions for in vitro cancer cell culture. Specifically, glucose oxidase (GOX) was acrylated and co-polymerized with poly(ethylene glycol)-diacrylate (PEGDA) through photopolymerization to form GOX-immobilized PEG-based hydrogels. We first evaluated the effect of soluble GOX on inducing solution hypoxia (O₂<5%) and found that both unmodified and acrylated GOX could sustain hypoxia for at least 24h even under ambient air condition with constant oxygen diffusion from the air-liquid interface. However, soluble GOX gradually lost its ability to sustain hypoxia after 24h due to the loss of enzyme activity over time. On the other hand, GOX-immobilized hydrogels were able to create hypoxia within the hydrogel for at least 120h, potentially due to enhanced protein stabilization by enzyme 'PEGylation' and immobilization. As a proof-of-concept, this GOX-immobilized hydrogel system was used to create hypoxia for in vitro culture of Molm14 (acute myeloid leukemia (AML) cell line) and Huh7 (hepatocellular carcinoma (HCC) cell line). Cells cultured in the presence of GOX-immobilized hydrogels remained viable for at least 24h. The expression of hypoxia associated genes, including carbonic anhydrase 9 (CA9) and lysyl oxidase (LOX), were significantly upregulated in cells cultured with GOX-immobilized hydrogels. These results have demonstrated the potential of using enzyme-immobilized hydrogels to create hypoxic environment for in vitro cancer cell culture.

Astrocyte-derived interleukin-15 exacerbates ischemic brain injury via propagation of cellular immunity

Astrocytes are believed to bridge interactions between infiltrating lymphocytes and neurons during brain ischemia, but the mechanisms for this action are poorly understood. Here we found that interleukin-15 (IL-15) is dramatically up-regulated in astrocytes of postmortem brain tissues from patients with ischemic stroke and in a mouse model of transient focal brain ischemia. We generated a glial fibrillary acidic protein (GFAP) promoter-controlled IL-15-expressing transgenic mouse (GFAP-IL-15^tg) line and found enlarged brain infarcts, exacerbated neurodeficits after the induction of brain ischemia. In addition, knockdown of IL-15 in astrocytes attenuated ischemic brain injury. Interestingly, the accumulation of CD8⁺ T and natural killer (NK) cells was augmented in these GFAP-IL-15^tg mice after brain ischemia. Of note, depletion of CD8⁺ T or NK cells attenuated ischemic brain injury in GFAP-IL-15^tg mice. Furthermore, knockdown of the IL-15 receptor α or blockade of cell-to-cell contact diminished the activation and effector function of CD8⁺ T and NK cells in GFAP-IL-15^tg mice, suggesting that astrocytic IL-15 is delivered in trans to target cells. Collectively, these findings indicate that astrocytic IL-15 could aggravate postischemic brain damage via propagation of CD8⁺ T and NK cell-mediated immunity.

Impact of Oxygen Levels on Human Hematopoietic Stem and Progenitor Cell Expansion

Oxygen levels are an important variable during the in vitro culture of stem cells. There has been increasing interest in the use of low oxygen to maximize proliferation and, in some cases, effect differentiation of stem cell populations. It is generally assumed that the defined pO₂ in the incubator reflects the pO₂ to which the stem cells are being exposed. However, we demonstrate that the pO₂ experienced by cells in static culture can change dramatically during the course of culture as cell numbers increase and as the oxygen utilization by cells exceeds the diffusion of oxygen through the media. Dynamic culture (whereby the cell culture plate is in constant motion) largely eliminates this effect, and a combination of low ambient oxygen and dynamic culture results in a fourfold increase in reconstituting capacity of human hematopoietic stem cells compared with those cultured in static culture at ambient oxygen tension. Cells cultured dynamically at 5% oxygen exhibited the best expansion: 30-fold increase by flow cytometry, 120-fold increase by colony assay, and 11% of human CD45 engraftment in the bone marrow of NOD/SCID mice. To our knowledge, this is the first study to compare individual and combined effects of oxygen and static or dynamic culture on hematopoietic ex vivo expansion. Understanding and controlling the effective oxygen tension experienced by cells may be important in clinical stem cell expansion systems, and these results may have relevance to the interpretation of low oxygen culture studies.

Adipogenesis, lipogenesis and lipolysis is stimulated by mild but not severe hypoxia in 3T3-L1 cells

In-vitro investigation of the effects of hypoxia is limited by physical laws of gas diffusion and cellular O2 consumption, making prolonged exposures to stable O2 concentrations impossible. Using a gas-permeable cultureware, chronic effects of mild and severe hypoxia on triglyceride accumulation, lipid droplet size distribution, spontaneous lipolysis and gene expression of adipocyte-specific markers were assessed. 3T3-L1 cells were differentiated under 20%, 4% or 1% O2 using a gas-permeable cultureware. Triglyceride accumulation, expression of genes characteristic for advanced adipocyte differentiation and involvement of key lipogenesis enzymes were assessed after exposures. Lipogenesis increased by 375% under mild hypoxia, but dropped by 43% in severe hypoxia. Mild, but not severe, hypoxia increased formation of large lipid droplets 6.4 fold and strongly induced gene expression of adipocyte-specific markers. Spontaneous lipolysis increased by 488% in mild, but only by 135% in severe hypoxia. Inhibition of ATP-dependent citrate lyase suppressed hypoxia-induced lipogenesis by 81% and 85%. Activation of HIF inhibited lipogenesis by 59%. Mild, but not severe, hypoxia stimulates lipolysis and promotes adipocyte differentiation, probably through excess of acetyl-CoA originating from tricarboxylic acid cycle independently of HIF activation.

A Novel Chip for Cyclic Stretch and Intermittent Hypoxia Cell Exposures Mimicking Obstructive Sleep Apnea

Intermittent hypoxia (IH), a hallmark of obstructive sleep apnea (OSA), plays a critical role in the pathogenesis of OSA-associated morbidities, especially in the cardiovascular and respiratory systems. Oxidative stress and inflammation induced by IH are suggested as main contributors of end-organ dysfunction in OSA patients and animal models. Since the molecular mechanisms underlying these in vivo pathological responses remain poorly understood, implementation of experimental in vitro cell-based systems capable of inducing high-frequency IH would be highly desirable. Here, we describe the design, fabrication, and validation of a versatile chip for subjecting cultured cells to fast changes in gas partial pressure and to cyclic stretch. The chip is fabricated with polydimethylsiloxane (PDMS) and consists of a cylindrical well-covered by a thin membrane. Cells cultured on top of the membrane can be subjected to fast changes in oxygen concentration (equilibrium time ~6 s). Moreover, cells can be subjected to cyclic stretch at cardiac or respiratory frequencies independently or simultaneously. Rat bone marrow-derived mesenchymal stem cells (MSCs) exposed to IH mimicking OSA and cyclic stretch at cardiac frequencies revealed that hypoxia-inducible factor 1α (HIF-1α) expression was increased in response to both stimuli. Thus, the chip provides a versatile tool for the study of cellular responses to cyclical hypoxia and stretch.

Novel Double-Hit Model of Radiation and Hyperoxia-Induced Oxidative Cell Damage Relevant to Space Travel

Spaceflight occasionally requires multiple extravehicular activities (EVA) that potentially subject astronauts to repeated changes in ambient oxygen superimposed on those of space radiation exposure. We thus developed a novel in vitro model system to test lung cell damage following repeated exposure to radiation and hyperoxia. Non-tumorigenic murine alveolar type II epithelial cells (C10) were exposed to >95% O₂ for 8 h only (O₂), 0.25 Gy ionizing γ-radiation (IR) only, or a double-hit combination of both challenges (O₂ + IR) followed by 16 h of normoxia (ambient air containing 21% O₂ and 5% CO₂) (1 cycle = 24 h, 2 cycles = 48 h). Cell survival, DNA damage, apoptosis, and indicators of oxidative stress were evaluated after 1 and 2 cycles of exposure. We observed a significant (p < 0.05) decrease in cell survival across all challenge conditions along with an increase in DNA damage, determined by Comet analysis and H2AX phosphorylation, and apoptosis, determined by Annexin-V staining, relative to cells unexposed to hyperoxia or radiation. DNA damage (GADD45α and cleaved-PARP), apoptotic (cleaved caspase-3 and BAX), and antioxidant (HO-1 and Nqo1) proteins were increased following radiation and hyperoxia exposure after 1 and 2 cycles of exposure. Importantly, exposure to combination challenge O₂ + IR exacerbated cell death and DNA damage compared to individual exposures O₂ or IR alone. Additionally levels of cell cycle proteins phospho-p53 and p21 were significantly increased, while levels of CDK1 and Cyclin B1 were decreased at both time points for all exposure groups. Similarly, proteins involved in cell cycle arrest was more profoundly changed with the combination challenges as compared to each stressor alone. These results correlate with a significant 4- to 6-fold increase in the ratio of cells in G2/G1 after 2 cycles of exposure to hyperoxic conditions. We have characterized a novel in vitro model of double-hit, low-level radiation and hyperoxia exposure that leads to oxidative lung cell injury, DNA damage, apoptosis, and cell cycle arrest.

A New Approach for On-Demand Generation of Various Oxygen Tensions for In Vitro Hypoxia Models

The development of in vitro disease models closely mimicking the functions of human disease has captured increasing attention in recent years. Oxygen tensions and gradients play essential roles in modulating biological systems in both physiologic and pathologic events. Thus, controlling oxygen tension is critical for mimicking physiologically relevant in vivo environments for cell, tissue and organ research. We present a new approach for on-demand generation of various oxygen tensions for in vitro hypoxia models. Proof-of-concept prototypes have been developed for conventional cell culture microplate by immobilizing a novel oxygen-consuming biomaterial on the 3D-printed insert. For the first time, rapid (~3.8 minutes to reach 0.5% O₂ from 20.9% O₂) and precisely controlled oxygen tensions/gradients (2.68 mmHg per 50 μm distance) were generated by exposing the biocompatible biomaterial to the different depth of cell culture media. In addition, changing the position of 3D-printed inserts with immobilized biomaterials relative to the cultured cells resulted in controllable and rapid changes in oxygen tensions (<130 seconds). Compared to the current technologies, our approach allows enhanced spatiotemporal resolution and accuracy of the oxygen tensions. Additionally, it does not interfere with the testing environment while maintaining ease of use. The elegance of oxygen tension manipulation introduced by our new approach will drastically improve control and lower the technological barrier of entry for hypoxia studies. Since the biomaterials can be immobilized in any devices, including microfluidic devices and 3D-printed tissues or organs, it will serve as the basis for a new generation of experimental models previously impossible or very difficult to implement.